D flip-flops are one of major functions in finite state machines, which in turn is the critical part of control logic. It is known that the control logic of a microprocessor can occupy 20% of the processor's power. As more advanced architecture concepts, such as register renaming and out-of-order execution in a superscalar microprocessor, are used, the control logic Will likely be more complicated and its power dissipation will likely grow beyond this current level. In addition, to boost processor clock frequency, modern processors typically adopt superpipelined execution which uses D flip-flops. Enhancing the speed of D flip-flops can either enable a higher clock rate or allow more logic depths between two pipeline registers. These two objectives of low power and high speed provide a need in the art for circuits, such as D flip-flop circuits, which are energy efficient.